Fixes #51542

Adds a `git_panel.starts_open` setting for parity with `project_panel`.
When set to `true`, the git panel opens automatically in new workspaces
that don't have saved panel state.

The default is `false`, matching current behavior (non-breaking).

### Usage

```json
{
  "git_panel": {
    "starts_open": true
  }
}
```

### Implementation

Follows the same pattern as `project_panel.starts_open`:

1. `GitPanelSettingsContent` - added `starts_open: Option<bool>` to the
settings schema
2. `GitPanelSettings` - added `starts_open: bool` field
3. `GitPanel` Panel impl - overrides `starts_open()` to read from
settings

Release Notes:

- Added `git_panel.starts_open` setting to control whether the git panel
opens automatically in new workspaces (default: false)

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>